1. Field of the Invention
The present invention relates to a removably mountable electrothermal printing head for non-impact printing and to the mounting for positioning it on the moving carriage of a printer. Such a head is designed to produce, in response to ingoing electrical impulses, visible imprints on a recording medium which is electrically insulating and heat sensitive.
2. Description of the Prior Art
For effective utilization of the speed of modern systems of electronic information handling, it is essential to have available peripheral high-speed printing units, and it is also desirable that such printing units should be fairly noiseless, since they are normally located near the user, in offices or other working sites. While the use of printing devices depending on the ballistic impact of a character-bearing member against an inked ribbon in contact with the recording medium often involves speed limitations intrinsic to the system of mechanical actuation of the character-bearing member, with noise levels often reaching the limits of tolerability, introduction of impactless printing devices removes the above-mentioned drawbacks of noisiness and slowness.
Impactless printing devices are known which utilize the method of electrothermal printing. For example, U.S. Pat. No. 3,596,055 relates to an electrothermal printing unit having a printing head with a matrix of points or printing elements for generating characters to be printed. The matrix is composed normally of 35 elements, i.e. of five columns evenly spaced over the width of the character, each containing seven elements, the seven elements of each column being also evenly spaced over the height of the character to be printed. The elements are heated by resistances incorporated in them when they are subjected to an electrical potential difference.
Printing takes place character by character, a selection and control circuit providing for electrical excitation and heating only those points of the matrix required to form the character that is to be printed.
According to the patent above-mentioned, the operation of printing is performed with a head fixed relative to the recording medium, and consists of pressing the head, with the printing points warm, on to the heat-sensitive recording medium. This operation is therefore similar to a hot-stamping operation, and the imprint of each point on the recording medium is substantially the same size as the printing points. The movement of the head along a printing line is therefore intermittent, with a number of halts equal to the number of the characters constituting a line of print. These halts, however brief and alternating with very rapid movements from one printing position to the next, contribute intrinsically to slowing down the printing speed.
Another method of printing by points of the electrothermal type, to which reference is made in our U.S. Pat. No. 3,777,116, consists of using an electrothermal head having a single column of seven elements regularly spaced over the height of the character to be printed; the height of each of these printing elements is that of the printed point which it is desired to obtain on the recording medium, while the width of each element is much less than that of the printed point. This permits the head to move continuously along a line of print, the printed point resulting from the combined action of heating the printing element and from the movement of the latter.
Now if we consider a configuration of characters of the kind of those obtainable with a matrix of 7 .times. 5 elements such as those described, this latter head generates each character by five elementary printing acts, a control and selecting circuit taking care of actuating, with the required delays, the elements required for printing the desired character.
Although in comparison with the method described in the aforesaid U.S. Pat. No. 3,596,055 an increased number of elementary printing acts is required to form the characters, this last method permits higher printing speeds, since the head moves rapidly and without any interruption along a line of print. In addition, the arrangements for moving the head are considerably simplified, since a step-wise movement is no longer necessary.
The device of U.S. Pat. No. 3,777,116 has, however, some drawbacks relating to the individual printing elements and fitting them into a single head. The elements are composed of graphite and are built into a rectangular body connected to the control and piloting circuit; on a tapered portion is formed the printing surface. The elements are fitted in a single head, with the printing surfaces in contact with a ribbon of metallic material, supported by the head, which constitutes the common return line for the actuation circuits of all of the printing elements. The head is positioned so that the metallic ribbon is in contact with a heat-sensitive recording medium.
The contact resistances between the printing surfaces of each element and the metallic ribbon, the particular dimensions of the printing elements and finally the material used, all contribute to a considerable increase in the electrical power required for activation of the printing elements.
Moreover in thermal printers normally the thermal printing head is fixedly secured to the carriage which moves the head along a printing line of the recording medium; therefore the replacing of said head owing to break or wear thereof is generally an operation that cannot be carried out by the user but requires the call of skilled personnel as, normally, it involves unscrewing and securing screws, to disconnect electric connections and to restore them when the replacing operation is carried out and, finally, to adjust the head position with respect to the printing medium by means of suitable adjusting devices.
Furthermore, the use of a head fixedly mounted on the carriage does not confer thereon any flexibility about the printing character format for the user, namely it does not allow the user to select, for instance, inclined characters by the immediate positioning of the suitable head on the displacing carriage of the printer.